Cellular telephone communications have greatly advanced since the first system became operational in Japan in 1979. These first-generation systems were analogue. By the end of the 1980's the technology had moved to second-generation digital systems for two reasons. Firstly, analogue systems in use became incompatible with each other, and secondly, it was more economic to operate digital systems. In the early 1990's various second-generation systems were introduced. In Europe, GSM (Global System for Mobile Communications) was introduced. The United States introduced a second-generation system (AMPS) different from GSM, although it subsequently introduced a modified GSM.
GSM represents an interface between cellular phones (User Equipment: UEs) and a Core Network (CN). The interface includes a series of Radio Network Controllers (RNCs), with each RNC connected to a series of Base Stations (BSs), each of which in turn transmits and receives to UEs within its broadcast range. At any one time a UE sends and receives signals to one of the RNCs through only one of the BSs. As a UE moves around under GSM a “hard handover” takes place between one base station and another. The UE is constantly monitoring the power level of respective pilot channel signals received from the current BS and in-range BSs, and is sending resulting power comparison data in signals through the present BS to the RNC controlling the current BS. When the RNC determines that a signal from one of the in-range BSs is being received by the UE at a power level greater by at least a threshold amount over the power level of the current BS, the RNC directs the UE to start communicating with that one of the in-range BSs and to cease communicating with the current BS. The handover is called a “hard handover” because the UE ceases transmitting to the one BS for a very short interval before it starts transmitting to the new BS; there is no “soft” transition involving communication with both BSs simultaneously.
With the tremendous growth in cellular phone services and the increased demand to use them for “roaming”, the need for a universal standard became recognized. Technology was at the same time improving, so this “third-generation” system could incorporate improved technology, and integrate telecommunications features with those of Internet Protocol (IP) networks. UMTS primarily only differs from GSM in the communication between the RNC level and the UE level; the CN remains essentially the same as with GSM. Instead of communication of a UE through a single BS at any one time, UMTS uses UTRAN (Universal Terrestrial Radio Access Network) consisting of a network in which RNCs communicate with a series of nodes (nodeBs) simultaneously. Each nodeB supports one or more “cells” each of which has a signal coverage area, and the UE communicates with the cell or cells from a series of nodeBs.
Instead of handover of a UE from one BS to another BS as in GSM, mobility of the UE causes a continuing change in those nodeBs by which the UE communicates with the RNC, e.g. some are dropped as others are picked up. A “UMTS UE” is capable of receiving and transmitting through a communication path that involves simultaneous operation of all of the cells through which the UE passes, the UE and the RNC being at respective opposite ends of a communication path that uses the cells. UMTS results not only in a unified universal standard for UE manufacturers, but its signal transmission and reception diversity results also in improved signal quality. GSM is meanwhile still also in use, with UMTS and GSM existing side-by-side in many locations to allow persons with either type of UE to effectively use their mobile phones. In the CN, UMTS and GSM communications are similar; as indicated earlier, the two systems differ mainly in how communication proceeds between the RNCs and the UEs.
In comparison to the “hard handover” used in GSM, UMTS uses “soft handover”. There also exists what is termed “softer handover”, but for purposes of this invention it is sufficient to only describe “soft handover”. As mentioned above, a UMTS UE operates with a continuously-updated set of cells through which it is receiving and transmitting to its RNC. The criteria for cells entering and leaving what is termed the “Active Set” of cells is known. A general description is provided, for instance, in “Radio Network Planning and Optimisation for UMTS” by Laiho, Wacker and Novosad. Cell addition, removal or replacement are three of the six categories of “Events” recognized under UMTS. Addition of a cell is termed “an Event 1a”, removal of a cell is termed “an Event 1b”, and replacement of a cell with another cell is termed “an Event 1c”. The addition, removal or replacement of cells is each dependent on differential power-level measurements between pilot signals of respective cells. Such measurements are taken periodically by the UE and transmitted to its RNC for appraisal. If the RNC determines that the criteria is satisfied for adding, removing or replacing one of the cells through which the UE is communicating, the RNC sends a signal to the UE to take necessary actions to either start or terminate communication with a cell, or to both start communication with one cell while terminating communication with another. FIG. 1 shows the equations involved in determining the Events 1a and 1b, and indicates the factors involved in that determination.